Glp1 Insulin Su Dose Reduction

---
name: glp1-insulin-su-dose-reduction
description: Reduce the dose of basal insulin or sulfonylurea when starting or escalating a GLP-1 receptor agonist (Mounjaro/tirzepatide or Wegovy/Ozempic/semaglutide) to avoid hypoglycemia. Use when a clinician asks "how much to reduce insulin when starting Ozempic", "sulfonylurea dose reduction on tirzepatide", "hypoglycemia risk adding GLP-1 to insulin", or needs concrete reduction percentages before co-prescribing. Grounded in Mounjaro and Wegovy Canadian Product Monographs plus SURPASS-5 protocol.
---

# GLP-1 RA + Insulin / Sulfonylurea — Proactive Dose Reduction

## Purpose

Adding a GLP-1 RA to a patient already on a sulfonylurea or basal insulin **increases hypoglycemia risk**. The monographs mandate "consider a reduction" — this skill turns that into a concrete starting number and a monitoring plan.

## Activation

Trigger when:
- Starting a GLP-1 RA in a patient on SU and/or basal insulin.
- Escalating the GLP-1 RA dose in the same combination.
- Planning insulin withdrawal as glycemic control improves (see scenarios).
- Patient reports hypoglycemia after adding/escalating GLP-1 RA.

## Baseline Hypoglycemia Numbers to Calibrate Against

- SURPASS-5 (tirzepatide + basal insulin glargine): baseline glargine reduced by **20%** when HbA1c ≤ 8.0% at randomisation.
- SURPASS-4 (tirzepatide + glargine + SU background): severe hypo 0.5–1.1% on Mounjaro vs 2.4% on glargine alone; hypo < 3.0 mmol/L 9.9–13.8% on Mounjaro vs 21.6% on glargine alone.
- Wegovy STEP trials: hypoglycemia rare without concomitant SU/insulin.

## Starting Rules

### Sulfonylurea

- **Reduce SU by 50%** at GLP-1 RA start if HbA1c < 8.0%.
- **Reduce SU by 25–50%** if HbA1c 8.0–9.0%.
- **Maintain SU** if HbA1c > 9.0% (less urgent).
- Consider **stopping SU** altogether once GLP-1 RA is at maintenance and glycemia improved — SU is replaceable with safer agents (SGLT2i, metformin).
- Hypoglycemia with SU often outlasts a dose drop — glipizide / glimepiride half-lives are long.

### Basal Insulin

- **Reduce basal insulin by 20%** at GLP-1 RA start if HbA1c ≤ 8.0% (per SURPASS-5 protocol).
- **Reduce basal insulin by 10–20%** if HbA1c 8.0–10.0%.
- **Do not reduce** if HbA1c > 10% or significantly hyperglycemic; let titration drive reduction.
- Titrate further based on self-monitored fasting glucose targets.

### Prandial / Bolus Insulin

- Monograph note (Mounjaro): not studied with short/medium-acting insulin or dual formulations.
- In practice, if patient is on bolus insulin and GLP-1 RA is added (off-label combination), reduce bolus doses alongside basal.

## Monitoring

- Self-monitor blood glucose **daily minimum** during first 2 weeks and after each GLP-1 RA escalation.
- Tell patient to recognise hypo symptoms and carry fast-acting carbohydrate.
- Reassess at 2 weeks, 4 weeks, and each dose escalation.
- Consider CGM if available — underestimated hypo risk in SU-treated patients.

## Insulin Withdrawal Protocol (from published scenarios)

In stable, well-controlled patients on basal insulin who have reached GLP-1 RA maintenance:

- Reduce basal insulin gradually as glycemic improvement becomes evident.
- Withdraw insulin entirely when dose is **< 15 IU/day** and glycemia remains controlled (cardiorenal-protected patient benefits from metabolic simplification).

## Drugs That Do NOT Usually Cause Hypoglycemia

When GLP-1 RA is combined with these, **no pre-emptive reduction** is needed:

- Metformin
- SGLT2 inhibitors
- DPP-4 inhibitors (avoid — do not combine with GLP-1 RA)
- Thiazolidinediones

(Though always re-assess at each visit.)

## Rules & Constraints

1. **Always reduce the riskier agent first** — SU is higher-risk than basal insulin for severe hypo.
2. **Don't wait for hypo to happen** — pre-emptive reduction is the standard.
3. **SU half-life matters** — reducing doesn't remove overnight hypoglycemia risk for ~24 h.
4. **No dose reduction for metformin/SGLT2i** at GLP-1 RA initiation (unless GI AEs — see `glp1-gi-ae-pharmacological-rescue` for the metformin-diarrhoea interaction).
5. **In T2D, Wegovy** — discontinuation can **increase** blood glucose; monitor glucose when stopping too, not only when starting.
6. **Pediatric Wegovy + T2D** — safety data limited; specialist input recommended.

## Reference

- MOUNJARO Product Monograph, §4.1 Dosing Considerations, §7 Endocrine and Metabolism, §8 Adverse Reactions (Hypoglycemia tables).
- WEGOVY Product Monograph, §4.2 Patients with T2D, §7 Endocrine and Metabolism.
- SURPASS-5 protocol (baseline insulin 20% reduction at HbA1c ≤ 8.0%).
- Gorgojo-Martínez JJ et al. *J Clin Med* 2023;12:145 (published scenario 4 for insulin withdrawal).